Incineration control

ABSTRACT

The specification discloses and describes a method and means for determining concentration of evaporated solvent in gases emitted from a drying oven and delivered to an incinerator, by simple measurement of the temperature of gases constituting products of combustion exhausted from the incinerator. By maintaining the fuel input to the incinerator constant and also maintaining the temperature of the gases entering the incinerator constant, the variation of the temperature of the products of combustion emitted from the incinerator with respect to a standard value reflects the variation in the concentration of evaporated solvents supplied to the incinerator. A valve means, responsively controlled according to the temperature of the products of combustion emitted from the incinerator, functions to regulate the admission of a diluent, such as air, to the oven, thereby regulating the concentration of evaporated solvent to a substantially constant and safe value and effecting regulation of a uniform temperature of gas products of combustion at the outlet of the incinerator.

Unite States Patent 1 1 Wilt, Jr. et a1.

1 Mar. 4, 1975 1 INCINERATION CONTROL [75] Inventors: Charles R. Wilt,Jr.; Floyd L.

Schauermann, both of Pittsburgh,

[73] Assignee: Salem Corporation, Pittsburgh, Pa. [2 2] Filed: Sept. 13.1973 [21] Appl. No.: 396,668

3.314159 4/1967 Betz 3.472.498 10/1969 Price et a1 3.601.900 8/1971Erisman et al. 3,706,445 12/1972 Gentry 34/79 Primary Emminer-Kenncth W.Sprague Assistant Examiner-Larry l. Schwartz Armrnar. Agent, orFirm-Buell, Blenko &

Ziesenheim 1 ABSTRACT The specification discloses and describes a methodand means for determining concentration of evaporated solvent in gasesemitted from a drying oven and delivered to an incinerator, by simplemeasurement of the temperature of gases constituting products ofcombustion exhausted from the incinerator. By maintaining the fuel inputto the incinerator constant and also maintaining the temperature of thegases entering the incinerator constant, the variation of thetemperature of the products of combustion emitted from the incineratorwith respect to a standard value reflects the variation in theconcentration of evaporated solvents supplied to the incinerator. Avalve means, responsively controlled according to the temperature of theproducts of combustion emitted from the incinerator.

functions to regulate the admission of a diluent. such as air, to theoven, thereby regulating the concentration of evaporated solvent to asubstantially constant and safe value and effecting regulation of auniform temperature of gas products of combustion at the outlet of theincinerator.

10 Claims, 2 Drawing Figures lncmex Slur) In Solvent In Tl L12 0n Stop1A Air Pulled In with Strip PATENTEU 5 sum 2 5 2 Strip In Zone I Oven 38Solvent ln On Strip 1K Air Pulled In With Strip 1 INCINERATION CONTROLThis invention relates to apparatus for incinerating combustiblesolvents evaporated in an oven of a paint drying conveyorsystem tothereby effectively eliminate emission of obnoxious pollutants to theatmosphere, as well as to dilution control apparatus for regulatingsolvent concentration in the oven to a safe value.

in order to safeguard against explosion in the solvent evaporative zoneof an oven, such as are included in paint drying oven system, theNational Fire Protective Association recommends dilution of the solventvapor to 25% of the lower explosive limit (L.E.L.), that is 25% of thevolumetric concentration of solvent at which a gaseous mixture willexplode.

Usually, solvent dilution in an-oven is achieved by a fan or blowerpulling fresh air into the oven. Because of the heat required to heatthe fresh air drawn into the oven to oven temperatures, it is preferableto recircu-. late gases discharged from the incinerator back to the ovenfor dilution purposes partly because these gases are already heated andthe fuel requirement with respect to that for heating fresh air is thusreduced, and partly because the recirculated gases contain carbondioxide, nitrogen and water vapor, in addition to air, and these gasesare better diluents for preventing an explosion than is air alone.

An oven is designed for a fixed maximum amount of combustible solventand in order to limit the solvent concentration, within the oven, toapproximately 25% of the L.E.L., a fan or blower of correspondinglyappropriate capacity is required to insure that the proper amount ofdiluent is pulled into the oven. A safe rule is to provide 10,000standard cubic feet of air for each gallon of solvent, or at an airdensity of 0,075 lb./ft., 750 lbs. of air per gallon of solvent.Assuming a maximum solvent load of 100 gallons/hr., it follows that75,000 lbs. of air or diluent an hour will be required. Since solventweights may be taken as an average of 7.5 lbs/gallon, 100 gallons ofsolvent will weigh 750 lbs. Since the weight of solvent thus representsonly 1% of the total weight of air or diluent per hour, the solventweight may be neglected without seriously affecting the accuracy of thecalculations. Accordingly, under the assumed conditions of maximumsolvent load, the appropriate capacity of fan required to maintain asafe percentage concentration of solvent is 75,000 lb./hr. -I 0.075lb./ft. orl X 10 cu.ft. per hour.

When the oven is operated at a solvent input rate less than the fixedmaximum rate, the volume of diluent passing through the oven at themaximum fan capacity is more than that required to maintain the 25%L.E.L. If the diluent intake to the oven corresponding to the maximumfan capacity is maintained, therefore, the fuel requirement to heat thismaximum diluent intake to the oven will be increased with reduction insolvent concentration.

If it were possible to directly measure the amount of solvent beingevaporated in the oven, conceivably it would be possible to reduce thespeed of the fan pulling diluent into the oven, to thereby reduce thevolume of diluent passing through the oven, with a consequent saving offuel to heat the diluent with respect to that which would otherwise berequired. With present-day equipment, however, it is not practical todirectly measure the amount of solvent being evaporated in an oven.Moreover, present-day devices for measuring concentrations of solventvapor in a gas or gas mixture are not sufficiently reliable to be usedefficiently for control purposes.

We are aware of generally pertinent prior art patents, such as U.S. Pat.No. 3,472,498 issued Oct. 14, 1969 to H. A. Price, et al., and U.S. Pat.No. 3,706,445 issued Dec. 19, 1972 to Charles B. Gentry relating toincinerator control systems. These patents disclose apparatus forrecirculation of incinerator exhaust gases to an oven, such as a paintdrying oven, to reduce fuel requirements for the oven. However, they donot disclose any means for measuring or controlling the concentration ofevaporated solvent in the oven.-

It is difficult to hold the solvent load constant in a paint conveyordrying line because of variation in line speed, load surface area, orcoating thickness. The evaporated solvent concentration in apaint-drying oven may thus decrease substantially over a prolongedperiod of time, thus involving prolonged and increased fuel requirementsfor heating the excess volume ofdiluent in the oven.

Moreover, if the solvent concentration in the gas mixture exhausted froma drying oven and delivered to an incinerator decreases, it follows thatwith less heat released in the incinerator, in consequence of theburning ofa reduced weight or volume of solvent, additional fuel isrequired to be supplied to the incinerator to maintain the appropriatetemperature within the incinerator for effective combustion. Theevaporated solvent delivered to the incinerator has very high chemicalheat content of the order of 100,000 BTU. per gallon. Consequently, theloss of this heat, occasioned by a reduced volume of solvent deliveredto the incinerator, must be compensated for by heat furnished byadditional fuel supplied directly to the incinerator. The cost of thisadditional fuel is substantial over a period of time.

If the cost of the additional fuel were to be disregarded, it would bepossible to simply regulate the supply of fuel to the incineratorautomatically in direct response to variations in the temperature of thegaseous products of combustion emitted at the exhaust outlet of theincinerator. Moreover, if relialble solvent analyzers were availableconceivably such an analyzer could be arranged to automatically controlthe speed of the fan, which draws the diluent into the oven, so as toreduce the volume of diluent drawn into the oven with reduction in thepercentage of concentration of the solvent. However, due to the currentlack of reliable solvent concentration analyzers, the automatic controlof the volume of diluent in this manner is not feasible.

It is the purpose of this invention to provide a novel method andarrangement for automatically regulating the solvent concentration inthe gas mixture within a paint drying oven to a safe percentage of thelower explosive limit (L.E.L.) by controlling the weight flow of gasmixture from the oven to the incinerator as a function of thetemperature of the gas mixture comprising the products of combustionemanating from the incinerator at an exhaust outlet thereof.

It is moreover a purpose of this invention to provide a novel method andmeans for controlling the opera tion of an incinerator for combustion ofnoxious solvent vapors derived from a paint drying oven or other sourceto insure uniformity of operating temperatures thereof, withoutvariation of the normal supply of fuel the gas mixture delivered to theincinerator from an oven. Conversely, the temperature of the products ofcombustion emanating from the incinerator may be regulated to a constantvalue by regulating to a constant value the percentage concentration ofsolvent in the gas mixture delivered to theincinerator in consequence ofa variation of the weight of diluent in the gas mixture delivered to theincinerator. This principle may be demonstrated mathematically asfollows: I

The following five variables affect the performance of an incinerator:

1. Chemical heat (B.T.U.s) in solvent of mixed stream (m) delivered toincinerator 2. Weight of diluent in stream (m) 3. Temperature of stream(m) 4. Temperature of products of combustion stream emanating fromincinerator 5. Amount of fuel supplied by stream (f) directly toincinerator. Using the following quantities, heat and weight balancesmay be mathematically expressed:

Q Heat Content B.T.U./hr. above 0 R W Weight flowing lbs/hr. Cp SpecificHeat B.T.U./lb. R T Temperature Rankin H Chemical heat contentB.T.U./lb. d Diluent s solvent Heat Balance (Subscripts denote streaminvolved) But Q has two components, namely sensible heat by virtue ofthe stream temperature and chemical heat provided by the solvent.Rewriting (l) s e m pm m f f e c However, the following are constants,or essentially so, and do not affect the accuracy of calculations. Kidentifies constant.

pl" ill pc c Substituting and rewriting (Ill) s s m m m WIKI= c c vWeight Balance Substitute (V) in (IV) From (Vl), the remaining variablesare:

W Solvent Weight as it effects heat input W Diluent weight (By previousassumption solvent weight is eliminated) T Temperature of mixed stream(m) W, Weight of fuel T, Temperature of products of combustion Nowassuming the fuel input to the incinerator to be a constant value, thenWI: fuel Also, since the solvent load is fixed by the process, then:

stream Substituting in (VI), then:

K! s m m m luel f m ine!) c Accordingly three variables remain, namely:

W (Weight flowing in mixed stream (m)) T (Temperature of gases in mixedstream (m)) T, (Temperature of products of combustion stream Assumingthe temperature T,,, to be regulated to a constant, it follows fromequation (Vll) that the temperature T, may be controlled by varying thequantity W,,,, the weight of mixed stream (m). Similarly, if thequantity T,, is permitted to vary between limits, these limits will alsodetermine the variation of T If the value of K (solvent load) changesthe quantity W that is the weight flowing in the mixed stream, may becorrespondingly varied to maintain the quantity T,

(temperature of products of combustion stream (c)) constant ofsubstantially so.

In carrying out the above objects, we provide means for regulating therate of fuel input to the incinerator to a constant value, means forregulating the recirculation of gases to maintain a constant temperatureof the gas mixture at the inlet to the incinerator, and means forcontrolling the weight of the gas mixture at the oven outlet accordingto the temperature of the exhaust gases at the outlet of theincinerator.

A preferred arrangement for regulating the weight of the gas mixtureadmitted to the incinerator and thereby limiting to a safe value theconcentration of solvent in the oven is described hereafter and shown inthe accompanying drawings, wherein:

FIG. 1 depicts diagrammatically a paint drying conveyor line withradiant type solvent evaporation zone, and

FIG. 2 shows a modified arrangement with regard to recirculation of theproduct of combustion gases.

Referring to FIG. 1 of the drawings, the apparatus comprising the paintdrying conveyor system' includes a housing enclosing a series of spacedsolvent evaporation and curing oven zones, designated Zone 1, Zone 2 andZone 3. For brevity, additional oven zones are omitted and representedmerely by the broken line paralleling Zone 3. Oven Zone 1 has an inlet10 for a conveyor carrying a painted product to be dried. Also shown,diagrammatically, are an inlet 11 for solvent and an inlet 12 for air.Actually, the air enters the oven through oven inlet 10 with the productand the solvent enters as part of the product coating.

Associated with the oven Zone 1 is an incinerator 13 which is incommunication with the oven Zone 1 via ductwork 14 in which is includedablower or fan 15 for supplying the exhaust gas mixture from the ovensolvent evaporation Zone 1 to the incinerator 13. Connected to theincinerator 13 is a fuel line 16 having a valve 17 therein which isautomatically controlled to regulate the rate of fuel supply to theincinerator to a constant value. 1

Connected to the incinerator 13 is an exhaust gas outlet duct 18 whichdivides into two branches. One branch, designated 19 goes to a radiantbaffle 20 which physically surrounds the work in Zone 1 and radiatesheat to the work. The second branch of duct 18 is designated 21 andprovides passage for incinerator exhaust gases to succeeding oven Zones2, 3 etc. and via a return duct 22, including a blower fan 23, to theoven Zone 1.

Opening out of duct 19 are three branch ducts, designated 24, 25 and 29.Duct 25 returns or recirculates a portion of the gas products ofcombustion of the incinerator to the oven Zone 1 under the control of avalve 26 which is controlled responsively to the temperature at theincinerator inlet duct 14, by a suitable thermoresponsive device 27, soas to maintain the temperature of gases constant in duct 14.

The branch duct29 opening out of duct 19 supplies a portion of the gasproducts of combustion from the incinerator to the radiant baffle 20from which the flow continues to a duct 30 leading to a heat recuperator31. A portion of the heat from recuperator 31 may be recovered from thesystem via a duct 33 or returned to atmosphere via a duct 32.

For regulating the volume or weight of gases, supplied to the'radiantbaffle 20, a valve 34 is provided having two inversely operable valveelements 35 and 36. Valve 34 is controlled according to the temperaturein the radiant baffle 20 by a thermally-responsive de vice 37. Valveelement 35 is opened to increase the flow of gas mixture through duct 24to duct 30, with an increase in temperature in the radiant baffle 20,while valve element 36 closes to correspondingly reduce the proportionof gases supplied to the radiant baffle 20.

Conversely, upon a reduction of the temperature in the radiant baffle,valve element 35 is operated to reduce flow therepast so as to increasedirect flow through duct 29 to the baffle 20, while valve element 36 isopened to accommodate the increased proportion of gas flow therepastfrom the radiant baffle 20 to the recuperator 31. The temperature of theradiant baffle 20 is thus regulated to a substantiallyconstant'temperature.

Diagrammatically, Zones 1 and 2 and Zones 2 and 3 are shown separated byducts 38 and 39 respectively although the usual arrangement is for thezones to abut and be separated by internal partitions if regulated.Portions of the total quantity of gas containing products of combustionfrom the incinerator 13 flowing through duct 21 are diverted throughbranch ducts 40 and 41 to Zones 2 and 3 respectively, under theinfluence of the incinerator fan 15. The duct 40 is connected to theinlet of a fan 42 as is a duct 44 leading out of the Zone 2. A returnduct 45 connects the outlet of fan 42 back to Zone 2. The proportion ofgas recirculated from Zone 2 relative to that supplied from duct 40 isdetermined by valve '46 which is controlled by a thermallyresponsivedevice 47 which monitors the temperature of the gas returned to Zone 2via the duct 45. Thus with an increaseof temperature in return duct 45,valve 46 closes to reduce the flow of gas from duct 40 to the Zone 2.Conversely, with a decrease of temperature of gas in the return duct 45,valve 46 opens to increase the flow of gas from duct 40 to Zone 2.

In a similar manner, blower 43 supplies gas proportionally from a duct48 connected to Zone 3 and from duct 41 to the Zone 3 via a return duct49. A valve 50 in the duct 41 is controlled by a thermallyresponsivedevice 51 which monitors the temperature of gas in re turn duct 49.

The gases recirculated to Zones 2 and 3, as just described, flow fromthe several zones via branch ducts 52 and 53, respectively. to returnduct 22, where they are returned to Zone 1 by fan 23. Although most ofthe solvent is evaporated in Zone 1, minor amounts will be evaporatedin'Zones 2 and 3 and these must be returned to the incinerator via Zone1.

In accordance with the objectives of our invention, we further provide avalve 55 in the duct 14 between fan 15 and the incinerator 13, and athermallyresponsive device 56 which monitors the temperature in duct 18,at the outlet of incinerator 13, for controlling the valve 55.Thermally-responsive device 56 is effective responsively to an increasein the temperature of gases in duct 18 at the outlet of incinerator 13above a predetermined temperature, to cause valve 55 to be operatedtoward the open position, thereby increasing the flow of air drawn intoZone 1 via passage 12. Conversely, thermally-responsive device 56 iseffective, responsively to a decrease in temperature of gases in duct 18at the outlet of incinerator 13 below the predetermined temperature, tocause valve 55 to be operated toward the closed position, therebyreducing the flow of air drawn into Zone 1 via passage 12.

Let it be assumed that the system is in operation with a conveyorbearing painted products moving progressively through Zones 1, 2 and 3.Also, let it be assumed that valve 17 is operating to regulate to aconstant value the rate of fuel supplied to the incinerator and thatthermallyresponsive device 27 is functioning to regulate a constanttemperature in duct 14. Let it also be understood that the solventconcentration in the gases leaving Zone 1 and entering the incinerator13 is at a safe percentage of the L.E.L. and that the resultingtemperature of gases leaving the incinerator is controlled bythermally-responsive device 56.

If, now, the temperature of the gases in duct 18 at the incineratoroutlet rises above the predetermined temperature this is an indicationthat the concentration of solvent in the Zone 1 is increasing.Accordingly, valve 55 is opened to increase the flow of air into theZone via passage 12, thereby resulting in a reduction in the temperatureof gases in duct 18 to the predetermined temperature.

If the temperature of the gases in duct 18 at the incinerator outletfalls below the predetermined temperature, this is an indication thatthe concentration of the solvent in Zone 1 is reducing. Accordinglyvalve 55 is operated toward the closed position, thereby reducing theflow of air into Zone 1 via passage 12. In consequence, theconcentration of solvent in Zone 1 increases, with the result that thetemperature in duct 18 at the outlet of the incinerator is restored tothe predetermined temperature.

Referring to FIG. 2, a modified arrangement is shown whereincorresponding parts are designated by the same reference numerals as inFIG. 1. The arrangement in FIG. 2 differs from FIG. 1 in providing aduct 25', in place of duct 25, which by-passes Zone 1 and is connectedto duct 14 adjacent the inlet to fan 15. Also the thermally-responsivedevice 27 is connected to register ture of the gases immediatelyadjacent the inlet to the incinerator.

It will be seen that this invention provides a novel method andarrangement for determining and controlling evaporated solventconcentration in a paint drying oven or other source as well as forcontrolling the operation of an incinerator to insure a uniformtemperature of the gas products of combustion at the outlet of theincinerator notwithstanding variations in the solvent load or in thedegree of evaporated solvent concentration. It will furthermore be seenthat this invention provides an incineration control arrangement whichenables economical operation with respect to fuel requirements and whichalso maintains a substantially uniform and efficient operatingtemperature notwithstanding variations in the chemical heat releasedincidental to combustion of solvents.

While this invention has been described in connection with a radiantheat transfer type of drying oven, it will be understood that theinvention is equally applicable to an oven using convective heattransfer in the solvent evaporation zone, or to other sources of noxioussolvent vapors. Also, modifications may be made in the apparatusspecifically described within the terms of the following claims.

We claim:

1. In a drying oven of the type having at least one zone in whichsolvents are evaporated during the drying process and exhausted withother gases to an incinerator for incineration, the improvement whichcomprises:

a. means providing a constant rate of supply of fuel to the incinerator,b. means regulating the temperature of the gases emitted from the ovenduring the drying process to a constant value, and

0. means controlling the quantity of air admitted to the oven as adiluent for the solvents in accordance with the variation of thetemperature of the gases constituting the products of combustion of theincinerator exhausted therefrom.

2. In a drying oven according to claim 1, wherein conduit means isprovided for recirculating back to the oven a portion of the products ofcombustion of the incinerator, and wherein the temperature regulatingmeans of clause (b) comprises valve means in said conduit means andthermally-responsive means exposed to the gases emitted from the ovenwhich controls the operation of said valve means to so regulate thequantity of products of combustion of the incinerator admitted as asolvent diluent to the oven as to regulate to a substantially constantvalue the temperature of the gases emitted from the oven.

3. In adrying oven according to claim 1, wherein said oven comprises aradiant baffle to which gases comprising products of combustion of theincinerator are supplied, and temperature controlled means responsive tothe radiant temperature of the said baffle for so controlling theadmission of the gases comprising products of combustion of theincinerator to the baffle as to maintain a substantially constantradiant temperature.

4. In a drying oven according to claim 1, wherein the oven has aplurality of drying zones, and wherein a portion of the products ofcombustion of the incinerator are recirculated through subsequent zonesback to an initial zone.

5. In a drying oven according to claim 3, wherein said oven comprises aheat recuperating means, and said temperature controlled means comprisesvalve means controlling the admission of products of combustion of theincinerator to said heat recuperating means.

6. In a drying oven according to claim 4, wherein temperature controlledvalve means is provided for controlling the proportions of products ofcombustion of the incinerator supplied to the respective subsequentzones.

7. In a drying oven according to claim 4 wherein a blower is providedfor each subsequent zone to supply a mixture of recirculated products ofcombustion of the incinerator and zone gases from the corresponding zoneback to the same zone, wherein valve means regulates the flow ofproducts of combustion to the blower. and wherein temperature controlledmeans responsive to the temperature of the gas mixture returned to thezone by the blower controls said valve means to regulate the proportionof products of combustion relative to zone gases returned to the zone.

8. In a drying oven of the type having at least one zone in whichcombustible materials are evolved from the product being dried duringthe drying process and exhausted in a gaseous stream to an incineratorfor incineration, the improvement which comprises:

a. means providing a constant rate of supply of fuel to the incinerator,

b. means regulating the temperature of the gases emitted from the ovenduring the drying process to a constant value, and

' 0. means controlling the quantity of air admitted to the oven as adiluent for the combustible materials in accordance with the variationof the temperature of the gases constituting the products of combustionof the incinerator exhausted thereform.

9. In a drying oven according to claim 8, wherein conduit means isprovided for recirculating back to the oven a portion of the products ofcombustion of the incinerator, and wherein the temperature regulatingmeans of clause (b) comprises valve means in said conduit means andthermally'responsive means exposed to the gaseous stream emitted fromthe oven which controls the operation of said valve means to so regulatethe quantity of products of combustion of the incinerator admitted tothe oven as a diluent for the combustible materials therein so as toregulate to a substantially constant value the temperature of thegaseous stream emitted from the oven.

10. In a drying oven according to claim 8, wherein said oven comprises aradiant baffle to which gases comprising products of combustion of theincinerator are supplied, and temperature controlled means responsive tothe radiant temperature of the said baffle for so controlling theadmission of the gases comprising products of combustion of theincinerator to the baffle as to maintain a substantially constantradiant temperature.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,868,779 DATED 3 March 4, 1975 iNVENTOR(S) I CHARLES R. WILT, JR. andFLOYD L. SCHAUERMANN It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line ll, "system" should be --systems-.

Column 3, under the heading "HEAT BALANCE" line 44, should read (I) Q QQ Column 6, line 36, "thermallyresponsive" should read-thermallyresponsive-.

Column 8, Claim 8, line 42, "thereform" should read therefrom--.

Signed and sealed this 6th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks

1. In a drying oven of the type having at least one zone in whichsolvents are evaporated during the drying process and exhausted withother gases to an incinerator for incineration, the improvement whichcomprises: a. means providing a constant rate of supply of fuel to theincinerator, b. means regulating the temperature of the gases emittedfrom the oven during the drying process to a constant value, and c.means controlling the quantity of air admitted to the oven as a diluentfor the solvents in accordance with the variation of the temperature ofthe gases constituting the products of combustion of the incineratorexhausted therefrom.
 2. In a drying oven according to claim 1, whereinconduit means is provided for recirculating back to the oven a portionof the products of combustion of the incinerator, and wherein thetemperature regulating means of clause (b) comprises valve means in saidconduit means and thermally-responsive means exposed to the gasesemitted from the oven which controls the operation of said valve meansto so regulate the quantity of products of combustion of the incineratoradmitted as a solvent diluent to the oven as to regulate to asubstantially constant value the temperature of the gases emitted fromthe oven.
 3. In a drying oven according to claim 1, wherein said ovencomprises a radiant baffle to which gases comprising products ofcombustion of the incinerator are supplied, and temperature controlledmeans responsive to the radiant temperature of the said baffle for socontrolling the admission of the gases comprising products of combustionof the incinerator to the baffle as to maintain a substantially constantradiant temperature.
 4. In a drying oven according to claim 1, whereinthe oven has a plurality of drying zones, and wherein a portion of theproducts of combustion of the incinerator are recirculated throughsubsequent zones back to an initial zone.
 5. In a drying oven accordingto claim 3, wherein said oven comprises a heat recuperating means, andsaid temperature controlled means comprises valve means controlling theadmission of products of combustion of the incinerator to said heatrecuperating means.
 6. In a drying oven according to claim 4, whereintemperature controlled valve means is provided for controlling theproportions of products of combustion of the incinerator supplied to therespective subsequent zones.
 7. In a drying oven according to claim 4wherein a blower is provided for each subsequent zone to supply amixture of recirculated products of combustion of the incinerator andzone gases from the corresponding zone back to the same zone, whereinvalve means regulates the flow of products of combustion to the blower,and wherein temperature controlled means responsive to the temperatureof the gas mixture returned to the zone by the blower controls saidvalve means to regulate the proportion of products of combustionrelative to zone gases returned to the zone.
 8. In a drying oven of thetype having at least one zone in which combustible materials are evolvedfrom the product being dried during the drying process and exhausted ina gaseous stream to an incinerator for incineration, the improvementwhich comprises: a. means providing a constant rate of supply of fuel tothe incinerator, b. means regulating the temperature of the gasesemitted from the oven during the drying process to a constant value, andc. means controlling the quantity of air admitted to the oven as adiluent for the combustible materials in accordance with the variationof the temperature of the gases constituting the products of combustionof the incinerator exhausted thereform.
 9. In a drying oven accoRding toclaim 8, wherein conduit means is provided for recirculating back to theoven a portion of the products of combustion of the incinerator, andwherein the temperature regulating means of clause (b) comprises valvemeans in said conduit means and thermally-responsive means exposed tothe gaseous stream emitted from the oven which controls the operation ofsaid valve means to so regulate the quantity of products of combustionof the incinerator admitted to the oven as a diluent for the combustiblematerials therein so as to regulate to a substantially constant valuethe temperature of the gaseous stream emitted from the oven.
 10. In adrying oven according to claim 8, wherein said oven comprises a radiantbaffle to which gases comprising products of combustion of theincinerator are supplied, and temperature controlled means responsive tothe radiant temperature of the said baffle for so controlling theadmission of the gases comprising products of combustion of theincinerator to the baffle as to maintain a substantially constantradiant temperature.